To determine the concentration of limewater solution

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Introduction

Chemistry -AS - Assessed Practical (Skills P & A) To determine the concentration of a limewater solution Planning Limewater is calcium hydroxide in solution. As it's a metal hydroxide, it is slightly alkaline and sparingly soluble in water, which is why it is weakly alkaline and has a pH of 9 to 10. Limewater is used as a test for carbon dioxide gas - carbon dioxide turns limewater cloudy. The cloudiness is due to a precipitate of calcium carbonate being formed. To determine the concentration of the limewater solution (in g dm-3) we can carry out an experiment. This experiment is a simple acid-base titration, in which we are given the concentration of hydrochloric acid which has a concentration of exactly 2.00 mol dm-3. Also we are provided with 250cm3 of limewater which has been made such that it contains approximately 1g dm-3. A titration is basically a neutralization reaction where we record the volume needed to completely react the unknown chemical with the known one. For titration, the concentration of the limewater is to be ascertained by reacting it with hydrochloric acid. The equation for this reaction is shown below: Ca(OH)2 (aq) + 2HCl (aq) � CaCl2 (aq) + 2H2O (l) As you can see, it takes two moles of hydrochloric acid to completely neutralize one mole of calcium hydroxide. ...read more.

Middle

5. Repeat step 2. 6. Empty out the beaker and wash out and then transfer then solution from the flask to the beaker. The concentration of limewater is Mr of Ca(OH)2 = 40 + 2(16 + 1) = 74 Moles = Mass / Mr Moles = 1 / 74 Conc. approx = 1/74 mol dm-3 = 0.0135 mol. Therefore for the equation the acid is 2 � 0.0135 = 0.027mol. The 0.027 mol dm-3 solution of HCl is now prepared and ready to use. Titration Before doing the actual experiment, it is important to know about the systematic errors that could occur during the titration and hence can lead to biased results. I will carry out the following tasks before titrating to avoid any systematic errors: * Close the stopcock and with the aid of a funnel pour the acid solution upto the highest volumetric graduation. * Check the bottom of the burette for any air bubbles as this can cause systematic errors. * Before titration wash out the burette 2-3 times with the reagent to avoid concentration changes. * When reading the measurement off the burette, I will read it from the bottom of the meniscus to the top of a line. * Before reaching the end-point I will slow down the rate of flow and dispense partial drops for precise drops. ...read more.

Conclusion

of the limewater solution I will use the following calculations: The average volume used from the results is required and must be in dm3. To convert from cm3 to dm3, divide by 1000. For the calculations V will be used to represent this volume in dm3. Moles can be worked out using the equation: Concentration = Moles / Volume Moles = Concentration x Volume = 0.02 x V The reaction shows that two moles of HCl react with one mole of Ca(OH)2 so in the neutralisation reaction there must have been twice as many moles of HCl than Ca(OH)2 . Therefore moles of Ca(OH)2 must be (0.02 x V) / 2 = 0.01 x V Again using Concentration = Moles / Volume: Concentration of Ca(OH)2 = (0.01 x V) / 0.025 = 0.4 x V The number of moles in dm3 will be concentration x1 which is 0.4 x V x 1 = 0.4 x V. The mass of the Calcium Hydroxide in the solution can now be worked out using: Moles = Mass / Mr --> Mass = Moles x Mr Mr of Ca(OH)2 = 40 + 2(16 + 1) = 74 Mass of Calcium Hydroxide in dm3 = 0.4 x V x 74 Therefore the concentration of the Limewater in g dm-3 will be: 0.4 x V x 74 = ________ gdm-3 Bibliography * http://en.wikipedia.org/wiki/Bromothymol_blue * http://en.wikipedia.org/wiki/Titration * http://www.ausetute.com.au/dilucalc. ...read more.

Related GCSE Aqueous Chemistry essays

* Fill the conical flask with 25cm of limewater using the squeezing pump and pipette. * Add three drops of methyl orange to the limewater and mix until it turns a light yellow depending on the concentration of the methyl orange.

Then run exactly 2.5cm� of this into a 250cm� volumetric flasks. 4. Next add distilled water and make up 'to the mark'. 5. Then turn it up and down several times to ensure thorough mixing. 6. This dilution 2.5cm� into the volumetric flask will give me an acid which had a new concentration.

Note down the difference between the initial and final reading. You will now need to carry out your first real test. To do this first empty the contents of your conical and wash it with de-ionised water. Then measure out another 25cm� of Ca(OH)2 and pour it into the conical flask, and refill the burette.

as it would take too little acid in order to neutralise the limewater (Ca(OH)2). If the values of the results are too small, the burette's accuracy is not sufficient to produce reliable results for the concentration of the limewater when calculated.

Alternatively I could have used phenolphthalein however I felt that using this would detect the end point prematurely. Method 1. Create the HCl solution mentioned above. 2. Set up the titration apparatus. Place the burette in the wooden clamp with the white tile and conical flask on sitting on the stand.

chosen to use it Pipette The pipette will give me a good degree of accuracy and make it easier for me to measure out volumes correctly into the volumetric flask. Volumetric Flask This flask holds an appropriate amount of solution for the number of repeats I wish to do for the experiment.

to make sure that the HCl has mixed well with the distilled water. And also the amount of water will be measured carefully by filling it up to the 250cm3 mark again, making sure at the same time that the meniscus is at the top of the graduated mark.